Gas turbine engines are capable of higher efficiencies when operated at higher temperatures. However, operation of the engine at such higher temperatures may negatively affect the properties of metal components traditionally used in gas turbine engines. Even with the introduction of complex cooling systems, there remains a practical maximum operating temperature for gas turbine engines constructed primarily from metal alloys and, consequently, a ceiling on the efficiency of such engines.
One alternative to improve the efficiency of gas turbine engines is to use ceramic matrix composite (CMC) materials for certain components in the engine that have traditionally been formed from metal alloys. CMC materials are not as susceptible as metallic components to the degradation of material properties caused by the high operating temperatures that are desired to improve the efficiency of the engine. However, despite favorable thermal properties of the CMC material components, the CMC material components have an allowable stress which is an order of magnitude lower than the component formed from metal alloys, a high degree of stiffness, and a significantly lower thermal expansion rate than metallic components, leading to poor load distribution at transfer points. With these limitations, CMC material components cannot merely be substituted for equivalent metal alloy components of identical geometric structures and subjected to the same pressure loading without exceeding the allowable stresses of the CMC material.
Despite these limitations, the advantages of CMC materials in high temperature applications have led to their limited use in gas turbine components such as turbine blade track sealing segments. Circumferentially surrounding a rotating turbine blade wheel, a static blade track sealing shroud is designed to maximize the working air flowing through the turbine blades by minimizing the amount of air which leaks by the blade tips, thereby increasing the efficiency of the engine. Such sealing shrouds are frequently composed of a plurality of segments positioned around the turbine axis. Due to the segmented nature of the shroud, the shroud requires seals between the segments in order to block air from escaping the working air flow path through any potential segment-to-segment gaps.
A typical CMC sealing segment comprises a u-shaped component. The thin, flanged edges of the u-shaped sealing segment are machined with holes and slots for mounting pin attachment. While machining CMC materials is not desirable as they are susceptible to shorter lifespans due to recession in the hot, humid gas turbine environment, the u-shaped design requires machining of holes and, in particular, a slot to allow relative motion between the CMC sealing segment and metal alloy support structures due to different rates of thermal expansion between these materials. Additional machining of u-shaped CMC segments is required to support inter-segment seals. Further, using thin walls in the sealing segment subjects the CMC material to high edge loading stresses due to the small contact area between the CMC wall and the mounting pin. These high stresses severely limit any residual load capacity in the CMC material such that it is limited to use in low pressure applications.
There exists a need for novel CMC structures and mounting techniques which allow the use of CMC materials in high pressure, high temperature gas turbine seal segment applications.